Generally, an ultrasonic sensor is used for an obstacle detection system of a vehicle. Fore example, with reference to JP-2003-284182A, an ultrasonic sensor of a piezoelectric type which is manufactured by a MEMS (micro electro mechanical system) technology is disclosed.
In this case, the ultrasonic sensor has a thin-walled portion (semiconductor activated layer and insulating membrane layer) which is formed at a semiconductor substrate having a SOI (silicon on insulator) construction, and a piezoelectric oscillator where a strong dielectric is sandwiched between two electrodes (upper detection electrode and lower detection electrode). The piezoelectric oscillator is arranged to cover the thin-walled portion in such a manner that the electrodes are respectively arranged at the lower surface and the upper surface of the strong dielectric.
That is, the forming part of the thin-walled portion and the piezoelectric oscillator constructs a membrane structure where the semiconductor activated layer, the insulating membrane layer, the lower electrode, the strong dielectric and the upper electrode are sequentially stacked.
In the case where the piezoelectric-typed ultrasonic sensor is manufactured by the MEMS technology, a resonance frequency of a membrane structure which is constructed of the thin-walled portion and the piezoelectric oscillator may deviate from a desirable value due to unevenness in manufacture. Moreover, in the case of an ultrasonic sensor where the membrane structures are arrayed, there will be unevenness in the resonance frequencies of the membrane structures (that is, unevenness in sensitivities thereof).
Considering the above-described problem, it is proposed in JP-2003-284182A to adjust the resonance frequency of the ultrasonic sensor by applying a predetermined voltage between the two electrodes during the operation of the ultrasonic sensor. In this case, with the variation of a spontaneous polarization generated in the strong dielectric, the value of the physical property (for example, membrane stress and Young's modulus) related to the stiffness varies so that the resonance frequency varies. However, it is difficult to control the resonance frequency for a long time, because the state of the polarization gradually varies when the voltage is continuously applied.
Moreover, according to JP-2003-284182A, it is also proposed that the predetermined voltage is applied between the two electrodes before the operation of the ultrasonic sensor to beforehand change (polling) the spontaneous polarization of the strong dielectric so that the resonance frequency of the ultrasonic sensor is adjusted. However, in this case, the spontaneous polarization will become weak when being provided with ambient temperature, for example. Therefore, it is difficult to control the resonance frequency for a long time.
Furthermore, according to the methods in JP-2003-284182A, it is difficult to adjust if the material does not have the spontaneous polarization such as the strong dielectric. For example, the methods cannot be suitably used for a piezoelectric material such as aluminum nitride (AlN), zinc oxide (ZnO) or the like.
Moreover, according to the ultrasonic sensor having the membrane structure disclosed in JP-2003-284182A, buckling will be caused when the internal stress (in the state where piezoelectric oscillator does not vibrate) of the whole of the membrane structure constructed of the thin-walled portion and the piezoelectric oscillator is a compressive stress. Because the shape of the membrane structure in the state where the piezoelectric oscillator does not vibrate varies (i.e., is lack of reproducibility) due to buckling, there will also occur a variation in the deformation amount of the membrane structure when ultrasonic wave is received or sent. That is, there will have a variation in the sensitivity of the ultrasonic sensor.
Generally, for the ultrasonic sensor having the membrane structure, the internal stress (in the state where piezoelectric oscillator does not vibrate) of the whole of the membrane structure is adjusted to be a tensile stress or to be zero. Thus, the membrane structure in the state where the piezoelectric oscillator does not vibrate is maintained to be substantially flat, to reduce the variation in the deformation amount of the membrane structure when the ultrasonic wave is received or sent. However, because the internal stress of the whole of the membrane structure is adjusted to be the tensile stress or to be zero, the deformation amount of the membrane structure when the ultrasonic wave is received or sent will be small. That is, the sensitivity of the ultrasonic sensor will be low.